We propose to continue our research on the crystal structures of small biologically important molecules, with an emphasis on studies that will enhance our knowldege of the nature of intermolecular forces and their relationship to molecular geometry and reactivity. Of particular interest to us are interactions involving hydrogen atoms, which cover most of the surface areas of biological molecules and hence must be of primary importance in the reactions of biological systems. In efforts to increase the accuracy with which the electron distribution associated with hydrogen atoms can be determined, we plan to investigate crystal structure at very low temperature, using a new apparatus which is simple to operate, extremely stable, and does not expend cryogenic materials.